When wood is ground to produce fibres, the grinders typically used are grinders in which the blocks of wood are pressed against the surface of a rotary grindstone, simultaneously spraying water there to produce a pulp suspension. The most generally, the wood supply is implemented in pulp grinders on a discontinuous basis: one batch of wood at a time is fed into a feed chute, after which the wood in the feed chute is pressed by a cylinder and a piston against the grindstone. In order that the output would be as great as possible, two feed chutes, with cylinders, are usually arranged on the opposite side, of the grindstone. Consequently, when a feed chute is being filled, the grindstone is subjected to less load than when both the feed chutes are in the grinding step, and this causes both uneven loading and variation in the quality of the ground pulp. Further, the drawback of the discontinuous supply is that the output is smaller when the wood is fed in batches than when continuous grinding is used. Another problem in the discontinuous grinding is that the blocks of wood fed during the compression press more firmly against each other, which also results in variation between the production rates at the beginning and at the end of the grinding. Consequently, for example the freeness of the ground pulp is higher at the beginning of the compression, dropping toward the end of the furnace, even if the feed rate at the piston of the cylinder is adjusted to remain constant. For the same reason, the motor is loaded unevenly.
Previously known are also continuous grinders in which the continuous wood supply is based on moving feed chains on both sides of a feed chute and on the weight of the wood in the feed chute. Such a grinder is known, for example, from German Offenlegungsschrift 28,12,299. The drawback of the solution is that to provide the continuous wood supply and sufficient compression, the chains must be rather long, which in practice means that the feed chute must be up to 6-8 metres high. The contact surface between the chains and the blocks of wood that are being fed is thus sufficiently large, and the weight of the pile of wood simultaneously helps to press the wood against the feed chains for compression. Because of this, only an essentially upright feed chute can be used in the grinding process, which notably restricts the amount of wood that can be ground simultaneously. As a result, the capacity of the grinder is naturally smaller than in solutions where wood fed from two or more feed chutes can be ground simultaneously. Another problem in the high feed chute is that the blocks of wood may settle obliquely, which affects the grindstone and because of which the grinder must be sharpened unduly often in order to correct the obliqueness. Since the pressing force of the chains does not divide evenly between the blocks of wood in the feed chute, but in practice the blocks of wood that are the closest to the chains are fed at a higher feed rate than those in the middle of the feed chute, this affects the quality and may also cause the above obliqueness.
European Patent 266,582 teaches a solution comprising one or two auxiliary pistons in addition to the actual pressing piston, the auxiliary pistons being pushed by separate feed cylinders toward the grindstone when the actual pressing piston is moved to the starting position of its stroke to allow the feed of a new batch of wood. In this embodiment, a separate closing trap is to be pushed in between the actual piston to prevent the wood from moving backward with the piston. The closing trap is to be pushed between the piston and the wood as the piston presses the wood, whereby the closing trap would have to be pushed to its place under great compression. In practice, this is not possible since the frictional forces caused by the pressing force are so great that the solution is impossible to implement. Correspondingly, if the piston were pulled back first, the operation of the trap and the grinding process would be essentially impaired.